In telecommunications networks that utilize wavelength division multiplexing (WDM), widely tunable lasers enable transmission of information at different wavelengths. Many proposed network configurations require transmitters that can be tuned to transmit at any of N distinct wavelengths. Even in networks where the individual transmitter wavelengths are held fixed, tunable sources are desirable for maintaining stability of the wavelength. Also, because the same part can be used for any channel, a tunable transmitter is useful from an inventory control perspective.
One prior art tunable laser design, disclosed in U.S. Pat. No. 5,771,252, uses an external optical cavity. As disclosed therein, a laser diode is used in combination with a diffraction grating and a rotating mirror to form an external optical cavity. The diffraction grating is fixed. As the mirror is rotated, light propagating within the optical cavity is fed back to the laser diode. The feedback causes the laser diode to “lase” with a changeable frequency that is a function of the rotation angle of the mirror. Unless accounted for, the frequency of the laser may “mode hop” due to the distinct, spatial longitudinal modes of the optical cavity. It is desirable that the longitudinal mode spectrum of the output beam of the laser diode change without discontinuities. This condition may be satisfied by careful selection of the pivot point about which the mirror is rotated, whereby both the optical cavity length and the grating feedback angle can be scanned such that the single pass optical path length of the external optical cavity is equal to the same integer number of half-wavelengths available across the tuning range of the laser cavity. If this condition is satisfied, rotation of the mirror will cause the frequency of the output beam to change without discontinuities and at a rate corresponding to the rotation of the mirror. U.S. Pat. No. 5,319,668 also describes a tunable laser. Both U.S. Pat. Nos. 5,771,252 and 5,319,668 disclose an expression for an optical cavity phase error, which represents the deviation in the number of wavelengths in the cavity from the desired constant value as a function of wavelength. The expression for optical cavity phase error includes terms related to the dispersion of the laser and other optical elements. U.S. Pat. No. 5,771,252 teaches a pivot point whereby the cavity phase error and its first and second derivatives with respect to the wavelength all go to zero at the center wavelength. For all practical purposes, the two methods describe the same pivot point.
The grating-based external cavity tunable laser (ECLs) of U.S. Pat. No. 5,771,252 is a relatively large, expensive device that is not suitable for use as a transmitter in a large-scale WDM network. Because of the size and distance between components, assembly and alignment of the prior art ECL above is difficult to achieve. Known prior art ECLs use stepper motors for coarse positioning and piezoelectric actuators for fine positioning of wavelength selective components. Because piezoelectric actuators exhibit hysteresis, precise temperature control is needed. In addition, prior art ECL lasers are not robust in the presence of shock and vibration.
Another prior art tunable laser design utilizes a Vertical-Cavity Surface-Emitting Laser (VCSEL). In one embodiment of this device, a MEMS (micro-electro-mechanical-system) mirror device is incorporated into the structure of the VCSEL and is used to tune the wavelength of the laser. Wide tuning range has been demonstrated in such devices for operation around 830 nanometers, but so far the development of a reliable, high performance VCSEL at 1550 nanometers has proved elusive. This device is very difficult to build because the MEMS device must be physically incorporated into the structure of the VCSEL. Furthermore, development of the MEMS actuators in InP-based materials is a formidable challenge.
In other prior art, angular motors have been used in angular gyroscopes and as fine tracking servo actuators for magnetic heads for disk drives. In “Angular Micropositioner for Disk Drives,” D. A. Horsley, A. Singh, A. P. Pisano, and R. Horowitz, Proceedings of the 10th Int. Workshop on Micro Electro Mechanical Systems, 1997, p. 454-458, a deep polysilicon device is described with radial flexures extending from a central fixed column, and radial, parallel plate electrodes that effect rotation of less than 0.5 degree. Batch Fabricated Area Efficient Milli-Actuators, L.-S. Fan, et. al., Proceedings 1994 Solid State Sensor and Actuator Workshop, Hilton Head, p. 38-42 shows a rotary flexural actuator with what appears to be two central flexures from central supports; the rotational range is not given but appears to be small. Dual Axis Operation of a Micromachined Rate Gyroscope, T. Juneau, A. P. Pisano, and J. H. Smith, Proceedings 1997 Int. Conf. On Solid State Sensors and Actuators, V.2, pp. 883-890 describes a polysilicon, surface micromachined gyro, which has four radial springs supporting a central circular mass. The springs are supported on the outside, and have a small strain relief feature. The angular drive range is not specified, but appears to be small. All of these prior art devices provide limited angular range. These prior art devices completely fill a circular area in a plan view, thus making it difficult or impossible to arrange such an actuator to provide a remote pivot location, as is required by ECLs.
Tunable Distributed Bragg Reflector (DBR) lasers are currently commercially available, however, these lasers have a limited tuning range. Total tuning of about 15 nanometers and continuous tuning without mode hops over about 5 nanometers range is typical.
A tunable laser based on sampled grating DBR technology is presently available. The DBR device is tunable over about 50 nanometers, but the fabrication is difficult and the control electronics are complex, requiring four different control currents.
Another prior art approach to making a tunable laser is to fabricate multiple Distributed Feedback (DFB) lasers on a single chip and couple them together with an arrayed waveguide structure. Each DFB is fabricated with a slightly different grating pitch so that each lases at a slightly different wavelength. Wavelength tuning is accomplished by activating the laser that matches the particular wavelength of interest. The main problems with this approach are cost and insertion loss. Furthermore, fabrication of multiple lasers on the same chip with different operating wavelengths may require direct e-beam writing of the gratings. Also, if one wants to cover a very wide tuning range, the number of lasers required is prohibitively large.
Additionally, the multiple laser approach is lossy because coupling N lasers together into one output waveguide results in an efficiency proportional to 1/N.
What is needed, therefore, is a tunable laser that provides advantages over the prior art.